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    <title>ShellCheck: SC2243 – Prefer explicit `-n` to check for output (or run command without `[`/`[[` to check for success)</title>
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    <h1>SC2243 – ShellCheck Wiki</h1>
    <a href="https://github.com/koalaman/shellcheck/wiki/SC2243">See this page on GitHub</a>
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    <h2
id="prefer-explicit--n-to-check-for-output-or-run-command-without--to-check-for-success">Prefer
explicit <code>-n</code> to check for output (or run command without
<code>[</code>/<code>[[</code> to check for success)</h2>
<h3 id="problematic-code">Problematic code:</h3>
<div class="sourceCode" id="cb1"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb1-1"><a href="SC2243.html#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="cf">if</span> <span class="bu">[</span> <span class="st">&quot;</span><span class="va">$(</span><span class="ex">mycommand</span> <span class="at">--myflags</span><span class="va">)</span><span class="st">&quot;</span> <span class="bu">]</span></span>
<span id="cb1-2"><a href="SC2243.html#cb1-2" aria-hidden="true" tabindex="-1"></a><span class="cf">then</span></span>
<span id="cb1-3"><a href="SC2243.html#cb1-3" aria-hidden="true" tabindex="-1"></a>  <span class="bu">echo</span> <span class="st">&quot;True&quot;</span></span>
<span id="cb1-4"><a href="SC2243.html#cb1-4" aria-hidden="true" tabindex="-1"></a><span class="cf">fi</span></span></code></pre></div>
<h3 id="correct-code">Correct code:</h3>
<div class="sourceCode" id="cb2"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb2-1"><a href="SC2243.html#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="co"># Check that the command outputs something on stdout</span></span>
<span id="cb2-2"><a href="SC2243.html#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="cf">if</span> <span class="bu">[</span> <span class="ot">-n</span> <span class="st">&quot;</span><span class="va">$(</span><span class="ex">mycommand</span> <span class="at">--myflags</span><span class="va">)</span><span class="st">&quot;</span> <span class="bu">]</span></span>
<span id="cb2-3"><a href="SC2243.html#cb2-3" aria-hidden="true" tabindex="-1"></a><span class="cf">then</span></span>
<span id="cb2-4"><a href="SC2243.html#cb2-4" aria-hidden="true" tabindex="-1"></a>  <span class="bu">echo</span> <span class="st">&quot;The command had output on stdout&quot;</span></span>
<span id="cb2-5"><a href="SC2243.html#cb2-5" aria-hidden="true" tabindex="-1"></a><span class="cf">fi</span></span>
<span id="cb2-6"><a href="SC2243.html#cb2-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-7"><a href="SC2243.html#cb2-7" aria-hidden="true" tabindex="-1"></a><span class="co"># Check instead that the command succeeded (exit code = 0)</span></span>
<span id="cb2-8"><a href="SC2243.html#cb2-8" aria-hidden="true" tabindex="-1"></a><span class="cf">if</span> <span class="ex">mycommand</span> <span class="at">--myflags</span></span>
<span id="cb2-9"><a href="SC2243.html#cb2-9" aria-hidden="true" tabindex="-1"></a><span class="cf">then</span></span>
<span id="cb2-10"><a href="SC2243.html#cb2-10" aria-hidden="true" tabindex="-1"></a>  <span class="bu">echo</span> <span class="st">&quot;The command reported success&quot;</span></span>
<span id="cb2-11"><a href="SC2243.html#cb2-11" aria-hidden="true" tabindex="-1"></a><span class="cf">fi</span></span></code></pre></div>
<p>(if the command instead outputs "0" or "false", see <a
href="SC2244.html">SC2244</a> for integer and "boolean" comparisons)</p>
<h3 id="rationale">Rationale:</h3>
<p><code>[ "$(mycommand)" ]</code> is equivalent to
<code>[ -n "$(mycommand)" ]</code> and checks whether the command's
output on stdout was non-empty.</p>
<p>Users more familiar with other languages are often surprised to learn
that it is nothing like e.g. <code>if (myfunction())</code>, since it
does not care about what the command/function <code>return</code>s.</p>
<p>Using an explicit <code>-n</code> helps clarify that this is purely a
string operation. And of course, if the intention was to check whether
the command ran successfully, now would be a good time to fix it as in
the alternate example.</p>
<h3 id="exceptions">Exceptions:</h3>
<p>If you are familiar with the semantics of <code>[</code>, you can <a
href="ignore.html">ignore</a> this suggestion with no ill effects.</p>
<h3 id="related-resources">Related resources:</h3>
<ul>
<li>Help by adding links to BashFAQ, StackOverflow, man pages, POSIX,
etc!</li>
</ul>
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